The present invention is directed toward microelectronic devices with improved heat dissipation, methods of making microelectronic devices, and methods of cooling microelectronic devices.
The current trend in microelectronic device fabrication is to manufacture smaller and faster microelectronic devices for computers, cell phones, pagers, personal digital assistants, and many other products. All microelectronic devices generate heat, and rejection of this heat is necessary for optimum and reliable operation. As the speed and capacity of microelectronic devices has increased, the integrated circuitry of the devices has become smaller and more closely spaced, thereby generating more heat. Moreover, the cooling space within the microelectronic devices has become smaller. Accordingly, heat dissipation has become a critical design factor.
FIGS. 1A-1C schematically illustrate an existing method for dissipating heat from devices formed on a wafer 110. FIG. 1A is a schematic side view of the wafer 110, and FIG. 1B is a schematic side view of the wafer 110 thinned, for example, in accordance with the procedures disclosed in U.S. Pat. No. 6,180,527, assigned to the assignee of the present invention and incorporated herein by reference. Thinning the wafer 110 increases the surface area per unit volume of the wafer 110, and therefore the ability of the wafer 110 to reject heat. FIG. 1C illustrates a microelectronic device 100 including a portion of the diced wafer 110, such as a microelectronic die 140, and a heat sink 130 attached to the die 140. In operation, the heat sink 130 absorbs heat from the die 140 and dissipates the heat into the ambient air. In one embodiment, a cooling fan can be added to force air past the heat sink 130.
Another method for dissipating heat from the die 140 includes attaching a heat pipe (not shown) to the surface of the die 140. A heat pipe typically includes a closed, evacuated vessel with a working fluid inside. One end of the heat pipe is positioned to absorb heat from the die 140. The heat causes the fluid in the heat pipe to vaporize and create a pressure gradient in the pipe. This pressure gradient forces the vapor to flow along the heat pipe to a cooler section where it condenses, giving up its latent heat of vaporization. The cooler section of the heat pipe then dissipates the heat into the ambient air. The working fluid then returns to the end of the heat pipe proximate to the die 140.
The foregoing heat dissipation methods have several drawbacks. For example, attaching a heat sink, heat pipe, and/or cooling fan to the microelectronic device may substantially increase the weight and/or size of the device. Furthermore, the limited contact area between the die and the heat sink or heat pipe may limit the heat transfer between the devices.
The present invention is directed toward microelectronic devices with improved heat dissipation, methods of making microelectronic devices, and methods of cooling microelectronic devices. In one aspect of the invention, a microelectronic device includes a microelectronic substrate having a first surface, a second surface facing opposite from the first surface, and a plurality of active devices at least proximate to the first surface. The second surface has a plurality of heat transfer surface features. In a further aspect of the invention, the second surface has a projected area and a surface area, including the heat transfer surface features, that is greater than the projected area. In yet a further aspect of the invention, the heat transfer surface features are not configured to provide electrical communication between the microelectronic substrate and components external to the microelectronic substrate. In a further aspect of the invention, the heat transfer surface features are integrally formed in the second surface.
In another aspect of the invention, the second surface of the microelectronic substrate defines at least in part a thermal conductor volume. The microelectronic device further includes an enclosure member sealably coupled to the microelectronic substrate, and a thermal conductor disposed within the thermal conductor volume to transfer heat from the active devices. In a further aspect of the invention, the second surface has a plurality of recesses. The microelectronic device further includes a sealed heat transport system coupled to the second surface of the microelectronic substrate. The heat transport system has a cavity with a thermal conductor configured to transfer heat from the microelectronic substrate to a region external to the microelectronic substrate, and the thermal conductor is sealably excluded from the recesses.
In another aspect of the invention, a method for making the microelectronic device includes forming active devices at least proximate to the first surface of the microelectronic substrate, and removing material from the second surface of the microelectronic substrate to form heat transfer surface features. In a further aspect of the invention, the method includes forming at least one recess in the second surface of the microelectronic substrate, and disposing the thermal conductor in the at least one recess. The thermal conductor is not configured to provide electrical communication between the microelectronic substrate and external components. The method further includes sealably enclosing the at least one recess with the thermal conductor positioned to transfer heat from the active devices to a region external to the microelectronic substrate.
In yet another aspect of the invention, a method for cooling the microelectronic device includes providing a microelectronic substrate having a first surface, a second surface with at least one recess, and a plurality of active devices at least proximate to the first surface. The method further includes vaporizing at least some of a liquid positioned in the at least one recess as the liquid absorbs heat from the microelectronic substrate, and condensing at least some of the vaporized liquid by transferring heat away from the microelectronic substrate.